Frequency variation response system



Nov. 18, 1941.

G. GUANELLA 2,262,932

FREQUENCY VARIATION RESPONSE SYSTEM Filed March 2, 1940 2 Sheets-Sheet l INVENTOR ATTORNEY Nov. 18, 1941. G. GUANELLA 2,262,932

' FREQUENCY VARIATION RESPONSE SYSTEM Filed March 2, 1940 2 Sheets-Sheet 2 INVENTOR BY eusmv GZ/AMELLA ATTORN EY variations of a modulating signal wave.

Patented Nov. 18, 1941 FREQUENCY VARIATION RESPONSE SYSTEM Gustav Guanella, Zurich, Switzerland, asaignor to Radio Patents Corporation, a corporation of New York Application March 2, 1940, Serial No. 321,833 In Switzerland September 14, 1939 6Claims.

The present invention relates to means for and method of generating modulated high frequency oscillations, and has for its main purpose'to prevent undesirable frequency fluctuations of the carrier frequency.

' As is well known, the frequency of amplitude modulated oscillations produced by a vacuum tube generator may be subject to substantial fluctuations having various origins such as changes of the operating potentials, temperature and a simultaneous frequency or phase modulation of the oscillations in accordance with the Especially in the case of short waves, such frequency fluctuations cannot be avoided or may be suppressed only with great difliculties. Such fluctuations require an increased transmission band width of the apparatus and channels used for the transmission of the signals resulting in a considerable amount of interference and disturbing signals being received together with the useful oscillations. On account of this com paratively wide frequency pass range, the superheterodyne method of reception for such oscillations presents great difficulties in practice.

In my co-pending patent application, Ser. No. 208,366, filed May 17, 1938, there is described a rapidly acting automatic frequency control adapted to eliminate fluctuations of a received carrier wave and making it possible to keep the pass range or band width of the selective means in a receiver substantially within the limits of the frequency band occasioned by the signal modulation. This arrangement, while greatly improving the operation and results of a receiver, does not limit the frequency band or range of the signals generated at the transmitter and passed through a transmitting channel such as a wireless channel by radiation from an antenna or by wired wireless transmission through a co-axial cable or other transmission line.

Accordingly, an object of the present invention is the provision of means for and a method of substantially eliminating carrier frequency fluctuations of a generator for high frequency oscillations having a characteristic (amplitude, frequency, etc.) modulated in accordance with a lower frequency signal wave.

A more specific object is to provide a means and method of suppressing undesirable carrier frequency fluctuations of an amplitude modulated transmitter of the order of and in the rhythm .of the frequencies of the modulating signal.

undesirable additional frequency fluctuations may occur also due to various causes such as variations of the operating potentials or frequency distortion caused by non-linear frequency modulation. Such frequency fluctuations will result in unavoidable interference and distortion in the receiver.

Accordingly, a further object of the invention is the provision of means for and a method of substantially suppressing undesirable additional fluctuations of the carrier frequency in a frequency modulated transmitter.

Slow or progressive variations of the carrier frequency of a modulated oscillation can be eliminated in a known manner by the aid of a manual or automatic frequency or tuning control. Undesirable rapid frequency fluctuations equivalent to an additional frequency modulation in the rhythm of a modulating (audio, video) signal wave cannot be eliminated or minimized by the known arrangements due to the large time constant used and inherent in the known controls which are effective only for gradual or progressive rather than a periodic frequency variation.

By the present invention the disturbing periodic frequency fluctuations of a modulated high frequency carrier wave are substantially eliminated by the provision of an auxiliary receiver at the transmitter adapted to generate a regulating potential or current varying proportionately to the instantaneous deviationof the frequency with respect to a predetermined fixed value. This control potential is applied to a fre- In case of frequency modulated oscillations,

quency controlling element of the oscillation generator through a transmission circuit or network including circuit elements designed to readily pass substantially instantaneously the modulating such as audible frequency components in such a manner as to reduce the relatively rapid periodic fluctuations of the carrier frequency to negligibly small value.

The novel method and .further objects of the invention will become more apparent from the following detailed description taken with reference to the accompanying drawings forming part of this specification and wherein:

Figures 1 and 2 are block diagrams'illustrating basic arrangements according to the invention, and

Figures 3 to 5 represent circuit diagrams of practical embodiments of the invention.

Referring more particularly to Figure. 1, 0 represents a high frequency generator or oscillator producing a carrier wave modulated according to a signal wave and the frequency of which is normally subject to relatively rapid fluctuations in the rhythm of the signal wave variations. The portion e: of the carrier energy e1 supplied by the oscillator O is received by the aid of an auxiliary receiver R embodying frequency variation response means adapted to produce a current or potential uo which varies in proportion to the instantaneous deviation of the oscillator frequency w from a desired value we. In case of a modulated oscillation, the frequency wo corresponds to the desired carrier frequency and the potential an is applied as control potential as to a frequency controlling element of the generator oscillator 0. An instantaneous frequency deviation of the generator frequency w from the desired value we will then result in a control potential determined by the following expression:

wherein 01 is a constant.

On the other hand, each control potential no will result in a proportional change of the gentotal transmitting time through all the oscillatory and other circuits, amplifiers, etc., of the erator frequency according to the following expression:

=wo+wz+cauz (2) wherein 02 is another constant and we represents the total of the undesirable frequency deviations having various origins such as changing operating conditions, interfering signals or the like. Due to the action of the control potential it: a final frequency w of the generator results whose deviation from the desired value is determined as follows in accordance with the Equations 1 and 2:

wherein As is seen, this frequency deviation is (1+0) times smaller than the deviation 10; without use of the invention due to any cause or origin. Thus, with sufficiently high values of the constants c; and c2 employed in designing the controlling circuit, the undesirable frequency fluctuations may be kept within any desired small limit range from the carrier frequency.

The rapidity of the regulating action is determined by the building up periods of the oscillatory circuits of the generator and the receiver, by the additional transmission time through the entire control circuit as well as the time constants of any filter or smoothing circuits which may be provided. An essential characteristic of the present invention is the fact that these building up periods and time constants are never allowed to exceed the oscillating period of the frequency fluctuations to be suppressed; that is, filter or smoothing circuits are avoided incapable of passing audible or other modulating frequency components. In this manner it is possible to suppress undesirable frequency fluctuations of the generator frequency in the rhythm of the audible or other modulating frequencies by means of a continuous or fast acting automatic frequency control. In practice the time constant of the total control circuit should be at the most of the order of 1000 of a second to fulfill this requirement for both audible and video frequencies to insure compensation of rapid carrier frequency fluctuation at the rate or directly in accordance with the signal frequencies.

The omission or reduction of the smoothing or filtering of the control potential applied from the auxiliary receiver to the transmitter through the control system, while the latter comprises the time required for the building up of the control potential upon occurrence of a sudden frequency variation, provided the building up occurs according to a linear function with respect to time. It can be shown that a stable control action is based upon the condition:

T; r 71') C (u) afore-mentioned requirement can be fulfilled in an easy manner in practice.

The method of the invention is also suited for controlling oscillations having a frequency modulated according to a signal frequency wave m. In this case the desired frequency we is not constant but varies in the rhythm of the modulating wave in as shown by the following expression:

wherein wo represents the constant average or carrier frequency and ca is a constant which may be assumed as follows:

A continuous adjustment of the frequency w to this desired value is obtained if the sum of the potential at produced in the frequency demodulator stage of the auxiliary receiver and the modulating signal potential in to be transmitted is applied to the generator 0 as frequency controlv potential us. In Figure 1 this addition of the signals at to the demodulated potential produced in the receiver R is indicated by the dashed arrow, whereby:

uz=uo+ur=cz.(wnw) +u1 (8) and the generator in place of the uncontrolled frequency (+?Dz) will produce a frequency in view of Equations 2 and 8 as follows:

tion of the output signals of the frequency demodulator stage in most of the known receiving systems.

According to Figure 2 the amplitude A may be maintained at a desired value At by means of a special amplitude control '(AVC) potential oz. This control potential is produced by rectification of the oscillations in the receiver R and is applied to an amplitude controlling element of the generator in such a sense as to counteract the deviations of the amplitude from the desired value. The control potential 02 may be expresed as follows:

v2=ki.(AoA) (11) wherein k1 is a constant. The amplitude of the frequency modulated oscillations is then as wherein k2 is a further constant and AZ represents the total of the amplitude deviations to be reduced or eliminated. The deviation of the amplitude obtained by this regulation from the constant value is then as follows in view of Equations 11 and 12:

wherein From the above it is seen that with sufllciently 1 quency modulation, etc., may be reduced to a negligibly small value. In order to suppress the instantaneous amplitude fluctuations occurring in the rhythm of the modulating (audio, video, etc.) signals, filters or smoothing networks in the transmission path of the control potential blocking or suppressing the modulating frequency components are to be avoided. In order to prevent hunting within the control system for the rapid amplitude control, the following stability condition:

should be fulfilled, wherein T represents the total transmitting period for the AVG potential caused by all the elements in the AVG circuit, and T4 represents the time of building up of the amplitude control potential upon occurrence of a sudden amplitude variation, provided the building up follows a linear function.

If the oscillations produced by the generator 0 are amplitude modulated in accordance with a signal wave to be transmitted, the instantaneous amplitude deviations from the desired value may be reduced by means of. rapid amplitude control described hereinbefore by applying the sum of the signals in to be transmitted and the potential 120 proportional to the amplitude obtained after rectification in the control receiver as the total amplitude control potential 02 to the oscillator, whereby the desired amplitude is expressed as follows in case of linear modulation of the transmitter:

wherein A0 represents the constant amplitude of the carrier wave and k3 is a constantwhich may be assumed as follows:

If the signals to be transmitted v; are applied as indicated in dotted lines in Figure 2, the amplitude control potential will be as follows:

This amplitude deviates merely by a negligible value from the desired amplitude A; as shown in the following:

provided K has a sufliciently high value. All variations Az of the uncontrolled amplitude as caused by the supply voltage fluctuations, amplitude distortion due to non-linear modulation, etc., in this manner are without appreciable influence upon the actual amplitude A of the controlled oscillations. In other words, this amplitude control constitutes an inverse feed-back between the output and input of'the transmitter providing all the advantages thereof known in the art.

Several practical embodiments of the invention are shown in Figures 3 to 5 and will now be described in detail.

In the arrangement according to Figure 3, the oscillations to be controlled are produced by the aid of a magnetron M of known design comprising a pair of semi-cylindrical anodes A1 and A: enclosing a cathode or filament H and surrounded by a magnetic coil F adapted to produce a magnetic field substantially in the direction of the filament H. The oscillations generated are impressed upon a Lecher wire or standing wave system L1 connected to the anodes A1 and A2 and provided with a short circuit rod or conductor N for tuning it to the generator frequency in a manner well understood by those skilled in the art. The anode potential supplied by a battery or the like B2 is impressed upon the center point of the short circuit conductor N through a choke coil Q1, while the filament H is suitably heated such as by means of a separate filament battery B1. The modulating signal potential 141 which may be an audio or other signal wave is superimposed upon the anode potential by way of terminals l, 2 to effect amplitude modulation of the magnetron oscillations. If the frequency of the oscillations is equal to the desired (carrier) frequency we, a nodal point will occur at a point of the Lecher system as indicated at K. A pair of double diode rectifiers D1 and D2 are connected to the opposite wires of the Lecher system through condensers C1 to C4 at equally spaced distances from the nodal point K. The filaments of the double diodes are energized by a common battery Ba, and the anodes of each diode connected through resistors R1 and R2, respectively. As is understood, single diode or any other type of rectifier may be provided for the purpose of the invention. If the oscillator fretheir output load resistors R: and R4 connected in series in opposing polarity. Accordingly, the combined differential output potential u: will be zero if the oscillator frequency corresponds to the desired value-amplified output potentials us-uw being equal to each other-and will increase in either direction from zero in accordance with and in proportion to an increase or decrease, respectively, of the generator frequency with respect to the desired value. The potential U2 accordingly varies in proportion to the genorator frequency deviations from the desired value both in magnitude and sign and is utilized for controlling the oscillator frequency by energizing the magnet coil F after amplification by means of triode V; in addition to the steady energization by a battery B4. The polarity adjustment and connections of the system are such that the frequency variations caused by the potential uz counteract the initial frequency variation resulting in generation of the potential uz. Since the frequency of the magnetron oscillations is dependent upon the strength of the magnetic field the Equation 2 is fulfilled at least approximately; that is, a control potential produced by an instantaneous variation of the magnetron frequency will counteract this variation, thereby reducing any frequency variations to a minimum in accordance with the Expression 3.

Referring to Figure 4, there is shown a system for eliminating or reducing undesirable frequency fluctuations in a frequency modulated transmitter. The generator in the example shown comprises a breaking field oscillator V4 of the Barkhausen type to produce micro rays and having a Lecher wire system In terminating in a dipole antenna P1 connected to one side of its grid and anode electrodes in a manner Well known. Positive grid and negative plate operating potentials supplied by batteries Be and B1 are applied through a further Lecher wire system L2 connected to the opposite ends of the grid and plate of the tube V; in series with the choke coils Q2 and Q3, the wire system L: being terminated by a blocking condenser C5. Each of the plate and grid circuits further includes a secondary winding. of the transformer T for applying a modulating potential to be described in detail hereafter. G represents a constant frequency spherical oscillator of known design comprising a pair of metal shells connected through a condenser Cs and constituting together with a triode V5 an auxiliary oscillator whose frequency due to the low losses and invariant resonance characteristics is extremely constant. Bs represents the anode battery, R7 a voltage drop resistance and Re a grid leak resistance for this constant frequency or standard oscillator. The oscillations produced by this oscillator are received simultaneously with the oscillations produced by the generator 0 by the dipole P2 of the auxiliary or control receiver. The beats produced due to-the difference of the frequencies of both oscillations result in an alternating potential after rectification in the diode rectifier V9, and sufficient filtering by a net ork comprising series choke coils Q6 and Q1 an a parallel resistance Re. any time corresponds to th difference frequency between the high frequency oscillations superimposed and applied to the diode V9. Any variation of the frequency of the oscillations of the generator 0 thus causes a corresponding variation of the beat frequency which latter is amplified by means of an amplifier U and applied by respectively.

This alternating beat potential at way of coupling condensers C7 and Ca to a pair of resonant circuits comprising inductance coils J9 and J10 shunted by condensers Ca and C10, One of these resonant circuits is tuned slightly above and the other below the average beat frequency 1120 in such a manner that the signal amplitudes in these circuits, and in turn the potentials developed in the output circuits of a pair of vacuum tube rectifiers Va and V-: are equal to each other if the beat frequency is equal to the desired average value wo. The triode rectifiers V6 and V? are shunted by condensers C11 and C12 and provided with output resistors R11 and R12 connected differentially with a common plate supply source B9 between the cathodes of the rectifiers and the junction point of the resistors. The differential potential uo is derived by means of condenser C13 conheated in series with a further resistance R1: forming a potential divider across the anodes to the rectifiers. Upon deviation of the transmitting frequency from the desired value, the beat frequency in the detuned circuits exciting the rectifiers V6 and V1 is varied accordingly whereby the oscillation amplitudes in these circuits become unequal due to the fact that the frequency approaches the resonant frequency of one circuit while simultaneously receding from the resonant frequency of the other circuit. The rectified potentials are then also unequal whereby the differential potential uo corresponds to the instantaneous deviations of the transmitting frequency from the desired value both in sign and magnitude. If the terminals 3 and a were short circuited this potential would act as control potential uz applied after further amplification by means of a triode amplifier V3 to the primary of the transformer T superimposed upon the steady anode current supplied by battery B10. Since the frequency of the oscillator V4 is dependent upon the potentials on the grid and plate a corresponding variation of the frequency will ensue in accordance with the variations of the control potential 'uo. tuations of the oscillator frequency may be eliminated or reduced to a negligible value in substantially the same manner as described hereinbefore. If, as shown in the diagram, the sum of the potential uo produced by means of the auxiliary receiver and the modulating potential in applied by way of terminals l and 2 is used as controlling potential ua, then the instantanev ous transmitting frequency deviation will be automatically adjusted at any instant in accordance with the instantaneous value of the modulatingsignal in as follows from Formula 9. All undesirable frequency fluctuations, which without the special frequency control would have the instantaneous value 10:, are reduced according to Formula 10 to a negligible minimum. As is understood, the time constants of all the circuits in the control receiver should be kept low relative to the oscillation period of the disturbing frequencies to be suppressed. This applies especially to the building up periods for the resonant circuits C9-J9 and C1o-J1o which should have a minimum damping as pointed out hereinabove.

Referring to Figure 5, there is shown a system comprising an amplitude modulated generator embodying means for maintaining the carrier frequency at a constant value and a special amplitude limiting arrangement for the oscillations used for producing the frequency control potential. The oscillator 0 comprises a vacuum tube V11 having associated therewith a regenerative In this manner undesirable fiuc-.

system for producing sustained oscillations comprising an oscillatory tank circuit consisting of an induction coil J 4 shunted by a condenser C11 and connected between the grid and plate of the tube. The cathode is connected to a tap of the induction coil J14 in series with a space current source B11 and a high frequency choke coil Q6, both the to be superimposed upon a steady anode potential supplied by the battery B11. A further vacuum tube V11 acting as a control tube is shunted across the oscillator tube V12 having its plate and grid connected through a condenser C12 whose impedance for high frequency current is high compared with the impedance of the resistance R25. In this manner, the anode cathode path of the tube l2 constitutes an effective capacity which may be varied by controlling the amplification in accordance with an average grid potential applied through the resistance 25. By this electronic capacity the tuning of the generator may be varied within predetermined limits.

The oscillations received by the antenna P4 of the auxiliary or control receiver are applied through the primary J12 of a tuned input transformer having a secondary G15 shunted by a tuning condenser C15 to one of the grids of a mixing tube V13. The other grid of tube V1: is excited in accordance with a standard oscillation produced by a-separate oscillator tube V14 by way of coupling condenser C22 and grid leak resistance R16. The grid circuit of the auxiliary oscillator comprises an inductance coil J13, a biasing re-. sistance R15 and a piezo-crystal or other stabilizing device K, while the anode or plate circuit includes a parallel resonant circuit consisting of an inductance coil J16 shunted by variable condenser C16. B14 represents the anode current source by-passed by a condenser for high frequency currents in a manner well known. The feedback between the plate and grid circuits for maintaining sustained oscillations in the example shown is byway of the internal grid-anode capacity C30 as indicated by dotted lines in the drawing. It is understood that any other stabilized oscillator may be used for the purpose of this invention.

There is thus produced in the output of the modulating or mixer tube V1: which includes a load resistance R11 and a space current supply source B12 a component having a frequency equal to the difference between the frequency of the generator and the standard crystal oscillator. This difference frequency accordingly varies in proportion to the frequency fluctuations of the received oscillations and is applied to a control grid of a composite duo-diode multi-grid amplifying tube V15 by way of coupling condenser C23 and grid leak resistance R26. The amplified intermediate frequency oscillation is impressed from the plate of tube V15 upon one of the diode anodes of the tube through coupling condenser C26, whereby a further condenser C21 is charged through a resistance R21 to a potential proportional to the intermediate frequency amplitude. The condenser C21 is continuously discharged through a resistance R22 in such a manner that the potentialat this condenser is enabled to follow the relatively rapid signal amplitude fluctuations. This (AVC) potential is applied to another grid of the tube V adapted to control the signal amplification in such a manner as to counteract the amplitude fluctuations of the signal variations in the anode circuit. Thus, the amplitude fluctuations or modulation of the oscillations applied to the intermediate frequency resonant circuit C11J11 are substantially suppressed. Any other known limiting device or system may however be provided to obtain the same object as is understood. B15 represents the anode current supply source for the tube V15. J16-C16 is a secondary tuned circuit arranged in inductive coupling relation with the circuit J11-C11 and having its electrical center (center tap of inductance l8) connected to the high potential side of the circuit J11C11 through coupling condenser C29. In this manner the sum and difference of the primary and secondary potentials of the resonant circuits are set up at the opposite sides of the tuned secondary circuit which sum and difference potentials are rectified by the aid of a pair of diode rectifiers V16 and V11 shunted by load resistors R23 and R24 having their anodes connected to the opposite sides of the resonant circuit and having their cathodes connected through a condenser C22 in such a manner that a rectified diiferential potential characteristic of the instantaneous deviations of the carrier frequency produced by the oscillator 0 from a desired value both in magnitude and sign is applied to the grid of the variable reactance tube V12 to control the tuning of the oscillator frequency so as to counteract an initial frequency deviation. According to a feature of the invention, the regulating circuit for the frequency control should have a suiilciently low time constant to prevent suppression of fluctuating frequency components within the range of the modulating frequencies. In particular, the building up period for the bandpass filter Cl'IJ1'l -C18J18 should be sufficiently high or the pass range of the filter should be large with respect to the low frequency components of the transmitter frequency fluctuations. 6 A reaction upon the rapid frequency control due to the amplitude modulation is substantially avoided by the suppression of the amplitude variations by the tube V15.

In order to control the low frequency modulation, the intermediate frequency oscillations are further applied by way of a condenser C21 to a further diode anode of the tube V15. The thus obtained low frequency potential 126 after suflicient filtering by means of series resistance R19 and condenser C25 and after amplification by means of an amplifier W is superimposed upon the initial modulating potential 111 supplied by way of terminals 1, 8 and the sum v2==v6+v1 impressed as modulation potential upon the oscillator O. In thismanner the high frequency amplitude A coincides at all times with great accuracy with the desired value A: due to the rapid amplitude control as follows from formula 20; that is, in other words, all non-linear amplitude distortions or other disturbances A2 are reduced to a negligible minimum by a negative feed-back or modulation control of this type.

It will be evident from the foregoing that the invention is not limited to the specific circuit arrangements and details as well as steps described andvdisclosed herein for illustration, but that the underlying idea and principles of the invention are susceptible of numerous variations and modifications-coming within the broader scope and spirit of the invention as defined in the appended claims. The specificationand drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. The combination with a transmitting system comprising an oscillator for generating a short wave carrier frequency, said carrier being subject to undesirable fluctuations in frequency, a standing wave tuning system excited by said oscillator and adapted to produce a standing potential wave having a nodal point shifting to either direction from a normal position in proportion to deviations of the oscillator frequency above and below, respectively, a desired frequency, means to derive potentials from points opposite to and eq -distant from said nodal point, means for rectifying the derived potentials and for differentially combining the rectified potentials, a frequency control element for said oscillator, and further means for impressing the differential potentials upon said control element to counteract the frequency deviations of said oscillator.

2. The combination with a transmitting system comprising an oscillator for generating a short wave carrier frequency modulated in amplitude according to a signal frequency wave, said carrier being subject to undesirable fluctuations in frequency at the rate of said signal frequency, a standing wave tuning system excited by said oscillator and adapted to produce a standing potential wave having a nodal point shifting to either direction from a normal position in proportion to deviations of the oscillator frequency above and below, respectively, a desired frequency, means for deriving potentials from points opposite to and equi-distant from said nodal point, means for rectifying the derived potentials and for differentially combining the rectified potentials, a frequency control element for said oscillator, and further means for impressing the diiferential potential upon said control element to counteract the deviations of the carrier frequency to substantially instantaneously compensate said carrier fluctuations,

3. In a transmitting system comprising an oscillator, a standing wave system excited by said oscillator, said standing wave system adapted to produce a standing potential wave having a nodal point shifting to either direction from a normal position in proportion to deviations of the oscillator frequency above and below, respectively, a predetermined frequency, means for deriving a pair of potentials from points at opposite sides of and substantially equally spaced from said nodal point, means for rectifying the derived potentials and for differentially combining the rectified potentials to produce a control potential varying in sign and magnitude in proportion to frequency deviations of said oscillator from said predetermined frequency, control means responsive to electric potential for varying the frequency of said oscillator, and means for impressing said control potential upon said control means to counteract the frequency deviations of said oscillator.

4. A frequency variation response system comprising a standing wave system excited by radio frequency signals of varying frequency, said standingwavesystem adapted to produce a standing potential wave having a nodal point shifting to either direction from a normal position in proportion to deviations of the signal frequency above and below, respectively, a predetermined frequency, means for deriving a pair of potentials from points at opposite sides of and substantially equally spaced from said nodal point, means for rectifying the derived potentials, and further means for differentially combining the rectified potentials.

5. A frequency variation response system comprising a standing wave system excited by high frequency signals of varying frequency, said standing wave system adapted to produce a standing potential wave having a nodal point shifting to either direction from a normal position in proportion to deviations of the signal frequency above and below,respectively, a predetermined frequency, means for deriving a pair of potentials from points located at opposite sides of and substantially equally distant from said nodal point, means for rectifying the derived potentials, and further means for differentially combining the rectified potentials.

6. A frequency variation response system comprising a two wire system excited at one end by radio frequency signals of varying frequency and provided with reflecting means at the opposite end to produce standing waves, a pair of rectifiers with means for impressing thereon signal voltages derived from two pairs of tapping points of said system, said pairs of tapping points being located at opposite sides of and substantially equally distant from a potential nodal point of said system when the exciting frequency has a predetermined value, and means for differentially combining the rectified potentials,

GUSTAV GUANELLA. 

